The present invention relates to the detection of aerosols, such as smoke and combustion aerosols, and to an improved device for detecting such aerosols.
More particularly, the present invention relates to aerosol detection devices of the general type disclosed and illustrated in copending U.S. Pat. application Ser. No. 588,153, now U.S. Pat. No. 4,012,729 which application was filed on June 19, 1975, and is assigned to the assignee of this application.
Disclosed and illustrated in the aforementioned patent application is a device for detecting smoke and like aerosols in atmosphere being monitored characterized by being formed of outer and inner interconnecting ion chambers and having a single particle or ion source located within the inner ion chamber. The particle source is positioned to radiate charged particles emitted therefrom into the inner ion chamber as well as through the path of interconnection of the ion chambers into the outer ion chamber. Thereby, through this construction, two different regions of ionization are maintained through the use of a single particle source.
The outer ion chamber is arranged to communicate directly with the atmosphere being monitored so that smoke and combustion aerosols in the atmosphere enter therein and is designed to function as a sensing ion chamber, as described in the aforementioned patent application. The inner ion chamber, which is designed to function as a reference ion chamber, connects directly only with the outer ion chamber and otherwise is totally isolated from all outside atmosphere in order to avoid contamination of the particle source therein.
It is noted that the terms outer and inner are employed herein to signify, respectively, that the outer ion chamber connects or communicates directly with the atmosphere outside of the aerosol detection device which is being monitored while the inner ion chamber has no path of direct connection or communication with any atmosphere outside of that within the device itself.
A problem inherent in this type of aerosol detection device is, that due to the free and direct path of connection existing between the outer and inner ion chambers, smoke aerosols entering the outer ion chamber from atmosphere outside the device may penetrate the inner ion chamber through the path of ion chamber interconnection and thereby contaminate the particle source. Such contamination of the particle source is detrimental to the operation of the aerosol detection device and can cause unstable and unreliable performance. Another problem inherent with the construction of this device described above is that, should the radioactive particle source become detached from the position in which it is safely mounted within the inner ion chamber, it could fall out of the inner chamber and create a radiation hazard for people in the surrounding area.
Patents of general interest with regard to the state of the art were listed in aforementioned U.S. Pat. application Ser. No. 588,153. This list of patents is incorporated herein by reference. Additionally, the following U.S. patents, which further illustrate the state of the art, have been cited in the copending U.S. patent application: U.S. Pat. Nos. 2,436,084; 3,585,649; 2,981,840; and, 3,295,121.
Of these last listed patents, U.S. Pat. No. 2,981,840 to Nahmias is of interest for its showing of the use of an electrically conductive thin metal foil to form a window through which alpha particles can be radiated from one ion chamber into another. As will become apparent, however, the construction, operation and concept of the Nahmias device is totally distinct and different from the improved aerosol detection device according to the present invention described hereinafter.